System for pasteurizing or sterilizing foodstuffs utilizing microwaves

ABSTRACT

Foodstuff packaged in a material transparent to microwaves is conveyed continuously through an environment pressurized to 2.5 bar max so as to prevent the containers or wrappers bursting or being forced out of shape; in four steps, the temperature is raised quickly to a prescribed treatment value with high frequency microwave radiation, stabilized by applying a combination of lower power microwaves and hot air, held steady with hot air only, and then lowered by cooling. In a split-level cylindrical oven, the upper level consists in a pressure-and-temperature compensating chamber (4), a high speed heat chamber (5) with closely spaced microwave generators (24) ordered in longitudinal rows along its top and sides, the waveguides (24 min , 25) of which are directed at the containers or wrappers (C) of of foodstuff from above and beneath as they are indexed through on special pallets (S), and a stabilization chamber (5 min ) in which lower power microwaves combine with air drawn through heaters (20) and circulated by a fan (21) located at the closed end of the oven; the same air is used to heat a vertical chamber (6), through which the pallets descend on an elevator (D) before passing into a cooling chamber (7) on the lower level and proceeding toward the final outgoing compensating chamber (8). Once out of the oven, the pallets (S) are raised by a further elevator (A) to the upper level for emptying and reloading.

BACKGROUND OF THE INVENTION

The invention disclosed relates to a method of pasteurizing andsterilizing foodstuffs utilizing microwaves, and to an oven forimplementing such a process, that is, a new microwave process by whichto reduce or eliminate bacteria from foods for human and animalconsumption, and thus prolong the period for which they can bepreserved; both the method and the oven used for its implementationconstitute the subject matter of the application.

In addition to the long-standing hot air, hot water and vapor methods ofpasteurization (85° C.) and sterilization (121° C.), and to methods ofdeep freezing (-25° C.), the prior art also embraces a type of treatmentutilizing microwaves generated at the conventional frequency of 2.450MHz.

Traditional hot air, hot water and vapor methods are beset by severaldrawbacks, namely long exposure times, lack of continuity, a cookedeffect produced by the high temperatures adopted to speed up theprocess, non-uniform temperatures, deterioration of or damage towrappings, and unsatisfactory results in the case of solid foods, due tothe slow transfer of heat by conduction.

There are also drawbacks with the deep-freezing of foodstuffs, mostnotably the greater complexity and cost of equipment as compared withthat utilized for pasteurization and sterilization.

Microwave treatment of foodstuffs involves the application of heat byinducing molecular vibration throughout the entire mass of the ediblesubstance, and thus brings a number of theoretical advantages:pasteurization or sterilization can be achieved in a much shorter timethan is possible by applying heat directly; the organoleptic propertiesof the food are improved, as also is its appearance and that of itswrapping; labor costs are cut by virtue of the automation obtainable;the cooked effect is reduced; sterilization of solids can be achievedwithout the need to use regulating fluids, and pasteurization is madepossible without excessive damage occurring to wrappings as a result ofinternal pressure generated by evaporation and heat. The onlydisadvantage is that the wrappings utilized must be transparent, whollyor in part, to microwave radiation.

Notwithstanding these apparent advantages, the attempts made thus far atmicrowave pasteurization and sterilization on an industrial scale havenot been such as to develop a continuous pasteurization method wherebypackaged foodstuffs can be subjected to uniform and stable heatthroughout the entire area of the oven; neither has sterilization beensuccessful, inasmuch as the requisite temperature level tends to prduceinternal gas pressures of an order that often causes the wrapping orcontainer to explode.

Accordingly, microwave methods do not constitute a viable proposition atindustrial level.

The prior art as summarized above thus stands in need of considerableimprovement, the objects being to eliminate the aforementioned drawbacksbesetting microwave pasteurization, and to enable the use of microwaveradiation in effecting sterilization on industrial scale.

It will be discerned from the foregoing that the requirement exists fora new method of microwave treatment capable of being operatedcontinuously, of ensuring that each container or wrapper containing thefoodstuff will be invested with a constant, uniform heat, according tothe temperature selected, without bursting or becoming distorted (evenat high sterilization temperatures), and of turning out an end-productsubstantially fresher than is obtainable conventionally, in less timeand with less waste.

SUMMARY OF THE INVENTION

The stated objects are achieved by adoption of a microwavepasteurization or sterilization method using high frequency microwaves,say of 2.450 MHz; the liquid or solid foodstuff for treatment, batchedinto containers or wrappers completely transparent to microwaves, is fedinto and conveyed forward through and environment pressurized in such away as to set up a balancing pressure, maximum 2.5 bar for example, thatwill prevent the container or wrapper from bursting or distorting as aresult of the heat generated by high frequency radiation.

The product is exposed to microwaves from above and below for an initialhigh speed heat step, say, of 4-10 minutes duration, in which thetemperature rises to a prescribed level; this is followed by a secondstep that serves to stabilize the initial temperature rise, and involvesfurther application of microwaves from above and below, in this instanceless intense and accompanied by hot air, for example between 90° and125° C. and circulated for a period of approximately 2-4 minutes, to theend of obtaining an identical temperature internally of each wrapper orcontainer. A third step involves maintaining the temperature uniform forbetween 1 and 10 minutes by continued circulation of hot air. In thefourth and final cooling step, the product is invested with cold air insuch a way as to bring the temperature down to approximately half thelevel of the former steps, or at all events to a level that will ensurethere is no danger of the containers or wrappers bursting or distorting.

Pressure levels are compensated during passage of the product fromatmospheric to internal pressurized conditions, and viceversa, as alsoare temperature levels, particularly at the entry stage.

The oven used to implement the method disclosed consists in alongitudinal cylindrical tunnel type structure divided horizontally intotwo distinct levels: an upper level, through which the containers orwrappers filled with the foodstuff travel during the initial steps,arranged in rank-and-file on pallets and propelled step by step alongtracks by horizontal rods, and a lower level of smaller volume alongwhich the return passage is effected in the same fashion.

Single pallets bearing the product are propelled along the upper levelby pairs of reciprocating rods: a first pair of rods passing through ahinged airtight flap, which push the pallet into an initial pressurecompensating chamber prior to its entering the oven; a second pair ofrods effecting passage from the chamber through a further hingedairtight flap into the oven proper; and a third pair of rods by whichthe column of pallets is propelled through to the end of the heatgeneration and temperature stabilization steps of the process.

Maintained substantially at the temperature and pressure levels existingin the oven proper, and capable of accomodating the contents of onepallet, the compensating chamber is equipped with valves that admit anddischarge pressurized air, and a pair of tracks across which the singlepallets pass in a longitudinal direction from the tracks at the entryside onto those at the exit side. The compensating chamber leads into aresonant heat chamber served by medium-low power microwave generators(e.g. adjustable between 0 and 1200 W in the initial high speed heatsection) arranged in closely spaced rows along the top and side of thecylinder; in a preferred embodiment, the three rows uppermost will bespaced apart approximately 40 cm apart lengthwise, one at center and theother two at 45° on either side, with two additional rows set at 30°farther around the circumference of the cylindrical structure (whichwill be some 2 meters in diameter).

The generators of the top three rows have hollow wave-guides ofrectangular section that project down radially toward the product,whilst the waveguides of the side rows project radially at first and arethen diverted horizontally, converging ultimately at center. Beyond thehigh speed heat chamber in the stabilization chamber, use is made oflower power microwave generators (max 1000 W as against 1200 W), setfarther apart and in alternation, combined with hot air drawn in fromthe top part of the tunnel, above the level of the conveyed product, bya fan unit located at the end of the cylindrical structure (which willbe some 12 meters in length); the air is directed over a baffle througha battery of electric heaters positioned in the top of the tunnel, takenin by the fan and blown at the oncoming pallets, passing through adescending elevator that occupies a vertical chamber located between thestabilization chamber and the fan end of the oven. The elevator receivesone pallet-load of the product with each indexed step of the propulsionsystem, tranferred into position by reciprocating rods with pivotedcatches, and at the same moment releases the pallet occupying its bottomtier, which is transferred through a hinged airtight flap onto thetracks of the lower return level of the oven.

Passage through the flap is produced by further reciprocatinglongitudinal propulsion rods with pivoted catches that movelongitudinally across the bottom of the vertical chamber above alongitudinal baffle which, together with the flap, separates thevertical hot air chamber from the cooling chamber; the rods engage eachof the descending pallets in turn and propel them nose to tail backthrough the cooling chamber toward the exit of the oven.

Blown horizontally through the descent elevator, hot air is gatheredbetween the separating baffle at bottom and a further baffle above, anddirected back into the stabilization zone, passing beneath the oncomingpallets then rising around and between them to return toward theheaters. The bottom part of the oven serves to effect the coolingprocess. Pallets carrying the containers or wrappers, transferred fromthe bottom tier of the descent elevator and propelled along the track,are invested with cold air from transversely positioned fan unitslocated at bottom; the temperature of the product is brought down to50°-60° C. or thereabouts by the cold air stream, the cold draft beingfavored by transverse ridges formed in the pallet surface.

On reaching the end of the return stretch, the leading pallet is engagedby a pair of reciprocating rods with pivoted catches and transferredinto the outgoing compensating chamber, which is situated beneath theincoming chamber and provided similarly with inlet and outlet airvalves, tracks, and hinged airtight flaps; the pressure level in thischamber is identical to that existing in the oven, and the temperatureidentical to that of the cooling stage.

The single pallets are transferred by further push rods out of thecompensating chamber and onto a pair of longitudinal horizontal beltloops which index them toward the bottom tier of an ascending elevator(lying in the same plane as the bottom tier of the descending elevator),then raised ultimately to a position that coincides with the upper levelon which entry into the oven occurs, but is set back from the actualentry point.

The pallet is now indexed forward by a pair of push rods onto a pair oftracks forming part of the upper level, moving through a distancecorresponding to the depth of the ranks of containers or wrappers inwhich the foodstuff is batched, whereupon the single containers orwrappers themselves are picked up by suckers mounted to a swing arm, andset down in single file on a transverse runout line.

As the rods continue indexing forward, the empty pallet is filled again,rank by rank, from an infeed line disposed parallel to the runout andserved by an identical swing arm with suckers; refilled, the pallet ispropelled forward by the same rods into the upper compensating chamber.Advantages of the invention are: uniform heat treatment through thedifferent areas of the oven, and from container to container;stabilization to a prescribed temperature effected in short intervals oftime and space; instant adaptability to different production cyclesgiven by extensive adjustment in power output of the individualmicrowave generators; constantly repeated pasteurization, and norejects; ease and speed of sterilization on industrial scale, with nodeterioration either of the packaging or of the end-product; low runningcosts.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in detail, by way of example, withthe aid of the accompanying drawings, in which:

FIGS. 1, 2 & 3 are longitudinal vertical sections of the ovenillustrating the zones in which the various steps of the method areimplemented, respectively: infeed/runout andpressure/temperature-compensation (FIG. 1); high speed heat, temperaturestabilization and cooling (FIG. 2); again, heat and stabilization, andsubsequent holding of the stable temperature (FIG. 3);

FIG. 4 is the vertical cross section through IV--IV in FIG. 2,illustrating the high speed heat zone;

FIG. 5 is a horizontal longitudinal section through V--V in FIG. 1,illustrating the system of push rods operating on the upper level of theoven, externally at the runout and infeed station, and internally ofpressur- and temperature compensating chamber;

FIG. 6 is a longitudinal horizontal section through VI--VI in FIG. 1,illustrating the passage through the outgoing pressure- and temperaturecompensating chamber and the lower level of the runout station.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the drawings, 1 denotes the protective outer casing of the entireoven installation, for example, embodied in stainless steel; thisencompasses a cylindrical metallic skeleton structure 2, and attachedthereto, an internal sheet metal lining 2' shaped to form a cylindricalenclosure and lagged externally with heat insulating material 3. 4denotes the incoming pressure-and-temperature compensating chamber, thecapacity of which is equal to the contents of one pallet-load of theedible product.

5 denotes the initial high speed heat chamber, and 5' the chamber inwhich the temperature reached in the first chamber is stabilized, andinternally of which pasteurization or sterilization of the end-producteffectively takes place. 6 denotes the chamber in which stablepasteurizing or sterilizing temperature is held steady for a givenprescribed duration. 7 denotes the chamber in which the product iscooled, and 8 the outgoing compensating chamber, likewise able toaccommodate one pallet.

Single containers C of the edible product to be pasteurized orsterilized are conveyed toward the oven along an infeed line AL andtransferred into special pallets S, each one of which exhibits a bearingsurface fashioned in polycarbonate material transparent to microwaves, ateflon frame (likewise transparent), and a right longitudinal section ofribbed profile; the transfer is effected by a swing arm 9 with suckers.

The pallet S is made to slide along a pair of longitudinal tracks 10,also in teflon, by means which comprise a pair of longitudinallydisposed pneumatic cylinders 11 located one at either side of the tracksand associated in turn with a pivoted catch 12 in such a way as tocreate a pair of actuators that index the pallet S through alongitudinal path into the compensating chamber 4.

PI denotes a hinged flap affording entry to the compensating chamber 4and incorporating sections of track that align with a further pair oflongitudinal teflon tracks 13 inside the chamber 4 itself, thusproviding a continuation of the external tracks 10.

PU denotes a hinged flap at the exit end of the compensated chamber 4,embodied in similar fashion to the entry flap PI and aligned with a pairof longitudinal teflon tracks 14 running the entire length of the twochambers denoted 5 and 5'.

15 denotes a second pair of longitudinal push rods, operated bybelt-driven propulsion units 16 in this instance, that serve to transferthe pallets S from the compensating chamber 4 to the heat chamber 5 andstabilization chamber 5'; once inside the heat chamber 5, the pallets Sare propelled through the oven nose-to-tail by a third pair oflongitudinal push rods 17, also belt-driven, that impinge on thetrailing edge of the hindmost pallet.

D denotes an intermittently operated descent elevator occupying thevertical chamber 6, which consists in tiered pairs of bearers associatedwith respective mutually opposed chain loops and receives the pallets Sfrom the longitudinal tracks 14 of the upper level and transfers them toa further pair of longitudinal tracks 18, also teflon, running throughthe cooling chamber 7 on the lower level.

19 denotes a pair of actuators consisting in pivoted catches operated bya piston and a pair of rack mechanisms, that serve to separate theleading pallet S of the column and position it on the vacant pair ofbearers offered by the elevator D.

20 denotes a set of electrical resistance heater elements occupying aposition at the top of the oven in a restricted terminal part of thestabilization chamber 5'; air drawn by a fan unit 21 through theseheater elements is channeled through the center rear opening 22 of alongitudinal baffle 23 that extends rearward through the restricted zoneand down toward the fan unit 21. Blown by the fan into the verticalchamber 6, the air is made to circulate beneath the pallets S occupyingthe temperature stabilization chamber 5'. The option exists of utilizinga heat exchanger in place of the electrical resistances.

24 denotes one of a plurality of magnetron type microwave generatorsarranged in longitudinal rows along the top half of the cylindricalenclosure and passing through the sheet lining 2' and insulation 3 tothe interior; the generators 24 are radially disposed and distributedalong the entire length of the heat and stabilization chambers 5 and 5',the three rows uppermost (see FIG. 4) being associated with waveguides24' that project inward radially toward the containers C proceedingalong the upper level. The waveguides 25 are equipped with radialperforations and project inward radially for a brief distance beforeextending into horizontal stretches that continue inward beneath thepallets S carrying the containers C and converge at center.

26 denotes a longitudinal baffle at the bottom of the oven, separatingthe vertical chamber 6 from the cooling chamber 7; this baffle 26 iseffectively a continuation of the downwardly directed part of the topbaffle 23, and is broken by a further hinged flap 27 through which thepallets S pass into the cooling chamber. 28 denotes the cap enclosingthe end of the cylindrical structure 2. 29 denotes a pair oflongitudinal rods occupying the cooling chamber 7, interconnected attheir two ends and operated by a further pair of belt-driven propulsionunits 16, the rear ends of which carry a pair of pivoted catches 30designed to engage the pallet S occupying the bottom tier of the descentelevator D and transfer it through the hinged flap 27 and onto thetracks 18 of the lower level. 31 denotes a plurality of transverselydisposed cooling fan units forming part of a bank of fin-fan heatexchangers.

32 denotes a pair of pivoted catches associated with a pair ofreciprocating rods, that serve to transfer the pallets S from thecooling chamber 7 into the outgoing compensating chamber 8. PI' and PU'denote the respective hinged entry and exit flaps affording passage tothe pallets S into and from this chamber 8, which are identical in allrespects to those PI and PU of the chamber 4 above; likewise, 33 denotesa pair of longitudinal teflon tracks in the outgoing chamber 8 that areidentical to the tracks of the incoming chamber 4.

34 denotes a pair of catches operated by a power cylinder 35 (FIG. 6),serving to transfer the single pallets S from the outgoing compensatingchamber 8 onto a pair of longitudinal belt loops 36.

A denotes an ascending elevator, identical in basic embodiment to thedescent elevator D, which receives the pallets S with their load ofcooled containers C from the belts 36 and transfers them back to theupper level, where they are returned to the pair of tracks 10 firstmentioned by a pair of push rods 37 identical to the pair denoted 15.

38 denotes a swing arm with suckers identical to the arm 9 firstmentioned, by which the containers C filled with the now pasteurized orsterilized edible product are transferred rank by rank onto a runoutconveyor EV.

39 denotes a longitudinal and substantially diametral horizontal cavityby which the upper and lower chambers 5 and 7 are separated thermally toavoid the formation of hot spots. 40 and 41 denote two longitudinalhorizontal shelves, the first fashioned in stainless steel andconstituting the bottom wall of the upper multiresonant half of thecylindrical enclosure, the second situated beneath the first 40,providing the top wall of the cooling chamber 7; 42 are slabs ofinsulating material identical to that denoted 3.

Operation of the oven will now be described.

Containers or wrappers C of the edible product to be pasteurized orsterilized approach in single file on the infeed conveyor AL and aretransferred rank by rank onto a pallet S prior to entering thepressure-and-temperature compensating chamber 4, which will be chargedwith air at the same pressure as that inside the oven proper (max 2.5bar for example), to the end of balancing the internal pressuregenerated in the containers or wrappers (fashioned, say, from a filmmaterial transparent to microwaves) by exposure to microwave radiationas they pass along the chambers 5 and 5' in which pasteurization orsterilization occurs; temperatures of the order of 100° C. and more aregenerated in the initial heat chamber 5, e.g. 120°-140° C. in the caseof sterilization, and then stabilized and held steady through thefollowing chamber 5' with the aid of hot air.

Once the pallet of containers C has covered a given distance, say 10meters, taking between 4 and 10 minutes, enveloped through the finalstretch by hot air circulating at between 90° and 125° C., theprescribed stable treatment temperature will have been reached; thepallet S is now transferred onto the descent elevator D, and after afurther period of between 1 and 10 minutes moving down through thevertical chamber 6, still enveloped by hot air, passes into the lowlevel cooling chamber 7.

The cooling chamber is charged with cold air at a pressure identical tothat of the top chambers, which brings the temperature down to some50°-60° C.; at this juncture, any danger of the container or wrapperbursting or being forced out of shape can be discarded, and thepasteurized or sterilized end product can be returned to atmosphericconditions.

The pallet S is taken out through the appropriately conditioned outgoingcompensating chamber 8 and back to the top level, where the containersare set down in single file on the runout line EV.

Uniformity of the heat treatment is ensured according to the inventionby the positioning of the waveguides, encircling the product as itproceeds through the top chambers 5--5', and by the facility ofregulating power output of the single microwave generators; indeed, thegenerators will be computer controlled in a preferred embodiment, so asto permit of monitoring their output individually and effecting theappropriate adjustments.

Similarly, the temperature of the hot and cold air supplies will becomputer controlled.

The embodiment described and illustrated is by no means limitative. Forexample, the bottom sets of waveguides 25 need not necessarily convergeat the center of the oven, but might be staggered on either side ofcenter.

Similarly, the maximum pressure of 2.5 bar is intended as a safe valueto balance the pressure generated internally of the container orwrapper, at all events, in the case of general solid foods for humanconsumption; nonetheless this value might be varied to suit theproperties of different products for pasteurization or sterilization, tothe same end of ensuring that the containers or wrappers do not burst,collapse or otherwise deteriorate.

What is claimed:
 1. A system for pasteurizing or sterilizing packagedfood, including an oven using microwaves at 2.450 MHz and pressure up to2.5 bar, and arranged to prevent bursting or distortion of the packagingon the food, said oven comprising:means for heating the packaged food toa predetermined temperature initially at a high speed; means forstabilizing and holding the predetermined temperature reached by initialheating to sterilize or pasteurize the packaged food; means for coolingthe packaged food following sterilization or pasteurization; said ovenarranged as a horizontally disposed cylindrical structure splithorizontally by a horizontal shelf into distinct upper level and lowerlevel chambers, both pressurized, the upper level chamber of greatervolume than the lower level chamber; and conveyer means for continuouslymoving packaged food along said upper level chamber and said lower levelchamber of said horizontally disposed cylindrical structure; said systemfor pasteurizing and sterilizing further including pallet means, eachhaving a ridged longitudinal profile to facilitate cooling and arrangedto carry packaged food while being propelled on said conveyor means; alongitudinal incoming pressure-and-temperature compensating chamber,external to said oven and furnished with tracks along which pallet meanscontaining packaged food are propelled, and a longitudinal outgoingpressure-and-temperature compensating chamber, external to said oven andfurnished with longitudinal tracks along which pallet means containingpackaged food are propelled.
 2. A system for pasteurizing or sterilizingpackaged food, including an oven using microwaves at 2.450 MHz andpressure up to 2.5 bar, and arranged to prevent bursting or distortionof the packaging on the food, said oven comprising:means for impartingmicrowave heating to the packaged food to heat the food to apredetermined temperature initially at a high speed; means forstabilizing and holding the predetermined temperature reached by initialheating to sterilize or pasteurize the food; and means for cooling thepackaged food following sterilization or pasteurization; said system forpasteurizing or sterilizing further including a station for loadingpackaged food into said oven, arranged external to said oven, saidstation for loading food comprising an infeed line to carry containersof packaged food in single file; and means for propelling packaged food,including a first reciprocating push rod fitted with a pivoted catchmechanism.
 3. A system for pasteurizing or sterilizing packaged foodincluding an oven, using microwaves at 2.450 MHz and pressure up to 2.5bar, and arranged to prevent bursting or distortion of the packages onthe food, said oven comprising:means for heating the packaged food to apredetermined temperature initially at a high speed; means forstabilizing and holding the predetermined temperature reached by initialheating to sterilize or pasteurize the packaged food; means for coolingthe packaged food following sterilization or pasteurization; said systemfor pasteurizing or sterilizing further including pallet means forcarrying said packaged food; a longitudinal incomingpressure-and-temperature compensating chamber for receiving food,arranged external to the oven; a longitudinal outgoingpressure-and-temperature compensating chamber for dispensing sterilizedor pasteurized packaged food, arranged external to the oven and beneathsaid longitudinal incoming pressure-and-temperature compensatingchamber; a station for handling packaged food, arranged external to theoven, wherein said packaged food is transported on pallet means, saidstation for handling packaged food comprisinglongitudinal belt looparranged at said longitudinal outgoing pressure-and-temperaturecompensating chamber to transport sterilized or pasteurized packagedfood on said pallet means away from said longitudinal outgoingpressure-and-temperature compensating chamber, a pair of longitudinaltracks arranged at said longitudinal incoming pressure-and-temperaturecompensating chamber to transport packaged food on said pallet means tosaid longitudinal incoming pressure-and-temperature compensatingchamber, elevator means to incrementally raise packaged food on saidpallet means from said longitudinal belt loop to said pair oflongitudinal tracks, a second longitudinal pushrod for incrementallymoving said pallet means from said elevator means to said pair oflongitudinal tracks, and runout line means for collecting sterilized orpasteurized packaged food on said pallet means raised by said elevatormeans, said runout line means arranged above said elevator means.
 4. Asystem as in claim 1, further comprising a pair of third longitudinalpushrods internal to the longitudinal incoming pressure-and-temperaturecompensating chamber, said third longitudinal pushrods being arranged topropel said pallet means from the longitudinal incomingpressure-and-temperature compensating chamber into the oven.
 5. A systemas in claim 1, further comprising a high speed heat chamber, atemperature sterilization chamber and a pair of fourth longitudinalpushrods located in the oven immediately beyond the longitudinalincoming pressure-and-temperature compensating chamber, in the oven saidpair of fourth longitudinal pushrods being arranged to propel the palletmeans nose to tail through the high speed heat chamber and thetemperature stabilization chamber.
 6. A system as in claim 1, furthercomprising a vertical chamber containing a descent elevator having a toptier and a bottom tier, and a fifth longitudinal reciprocating pushrodwith a pivoted catch mechanism located in the oven, said fifthlongitudinal reciprocating pushrod being arranged to transfer palletmeans farthest forward on the conveyor means of the upper level chamberonto the top tier of the descent elevator in the vertical chamber.
 7. Asystem as in claim 1, further comprising a pair of sixth longitudinalreciprocating rods with pivoted catch mechanism, said sixth longitudinalreciprocating rods being arranged to remove individual pallet means fromthe bottom tier of the descent elevator and to propel said individualpallet means nose to tail along the conveyor means of the lower levelchamber.
 8. A system as in claim 1, comprising a pair of seventhlongitudinal reciprocating rods with pivoted catch mechanisms located atan end of the lower level chamber said seventh longitudinal pushrodsbeing arranged to transfer pallet means farthest forward along theconveyor means of the lower level into the longitudinal outgoingpressure-and-temperature compensating chamber.
 9. A system as in claim1, further comprising a pair of eighth longitudinal pushrods locatedinternal to the longitudinal outgoing pressure-and-temperaturecompensating chamber, said pair of eighth longitudinal pushrods beingarranged to transfer individual pallet means bearing containers orwrappers of the pasteurized or sterilized food from the longitudinaloutgoing pressure-and-temperature compensating chamber onto alongitudinal conveyor, said longitudinal conveyor including a pair ofbelt loops arranged external to the oven and the longitudinal outgoingpressure-and-temperature compensating chamber.
 10. A system as in claim1 further comprising a high speed heat chamber served by a plurality ofmedium-low power microwave generators singly adjustable for power outputbetween zero and a given maximum, said microwave generators being spacedapart longitudinally at close intervals along the top half of thehorizontally disposed cylindrical structure and arranged in three upperrows, disposed one at center and two at 45° on either side of centerwith waveguides projecting downward radially toward the packaged food onsaid pallet means, and two additional rows of microwave generatorspositioned 30° farther around an arc formed by the positions of thefirst three rows of microwave generators on either side of the firstthree rows of microwave generators, said two additional rows ofmicrowave generators having waveguides projecting inward radially fromalong said arc for a first part of the waveguide length and bent to bedirected horizontally for a second part of the waveguide length, saidsecond part of the waveguide length of said additional two rows ofmicrowave generators being arranged below the packaged food on thepallet means.
 11. A system as in claim 10, further comprising astabilization chamber served by a second plurality of microwavegenerators, spaced less closely along the top half of the horizontallydisposed cylindrical structure and of lower rated power than saidmedium-low power microwave generators, in combination with a fan unitinstalled at one end of the horizontally disposed cylindrical structure,said fan unit being arranged to propel air filling space above thepallet means along the upper level chamber, said air being drawn in overthe top side of a longitudinal baffle located high up in the upper levelchamber, directed through a battery of heaters, gathered by a furtherlongitudinal baffle located in the lower level chamber and then blownacross a vertical chamber into the stabilization chamber beneathoncoming pallet means in said lower level chamber from where said airrises back to the space above the pallet means.
 12. A system as in claim11 further comprising a longitudinal cooling chamber, separated from thevertical chamber by a hinged flap and furnished with a battery oftransversely disposed cooling fans from which cold air is blown at thepallet means proceeding along the conveyor means of the lower levelchamber toward the longitudinal outgoing pressure-and-temperaturecompensating chamber.